AUM
Originally, it is written as auṁ, with a diphthong. The three letters are a-u-ṁ.
To write auṁ in Devanagari, the commonly used symbol is employed.
The first part is 'a', the second part is 'u', and the third is the sign above, the chandrabindu, 'the dot above the moon', representing the nasal sound, called anunasika.
Some authors say that auṁ consists of 5 parts: a-u-ṁ-nasal reverberation and silence.
Let us take this opportunity to mention a very important pronunciation detail.
In Sanskrit, there are short and long vowels.
The long vowel ā has a horizontal line in transliteration and lasts twice as long as the short vowel. The most important thing is that the short vowel a is more closed, and the long one is more open.
The short vowel a resembles the neutral Catalan or French vowel. However, many people pronounce it as if it were open, which is incorrect. By pronouncing it incorrectly, they are changing the gender of the word (masculine words end in short a, feminine in long ā).
Listen to native speakers or those who pronounce it well and pay attention.
Source: Explanation from Elena Sierra, Sanskrit professor at the Universidad de Barcelona: https://www.youtube.com/watch?v=snK2xmwp5jY